1. Field of the Invention
The present invention concerns a device for processing a signal carrying information intended to be transmitted using a radiofrequency signal. It also concerns a radiofrequency transmission system including such a device and a corresponding method.
The invention applies more specifically to the radiofrequency transmission of data using wireless communication terminals such as mobile telephones, for example, and more specifically using wireless terminals capable of transmitting in a broad range of power ratings, and according to several separate transmission standards.
2. Description of the Related Art
Each standard, for example GSM, EDGE, WCDMA, HSDPA, 3G, 4G, WIFI, WIMAX, 802.11, 802.16, etc., transmits in a specific transmission frequency band in the band of frequencies of the radio waves. In addition each standard transmits with a particular modulation.
Thus, a radiofrequency transmitter suitable for transmitting according to a particular standard is not generally suitable for transmitting according to another standard. For example, the architecture of a radiofrequency transmitter according to the GSM standard is based on GMSK-type constant-amplitude direct modulation, and therefore generally includes an analog modulator, the output of which is connected to a power amplifier. Conversely, the architecture of a radiofrequency transmitter according to the EDGE standard is based on a 8-PSK-type modulation with non-constant amplitude, generally with a polar loop. But an architecture with a polar loop is restricted in terms of the bandwidth of the transmitted signals and has difficulty transmitting signals with greater bandwidth than that allowed by the EDGE standard. In addition, in the case of greater bandwidth transmissions, such as those designed for the 3G and 4G standards, using an OFDM-type modulation, a direct-modulation architecture with linear amplification is generally favoured.
At the current time, to manage all these standards and the incompatibility of their respective architectures, multi-standard terminals are designed using several transmission microelectronic devices, each one being dedicated to one particular standard. This short-term solution, consisting in incorporating several microelectronic transmission devices in a single multi-standard terminal, is costly and inefficient. It leads to a duplication of radiofrequency emission functions in a single terminal, which then becomes bulky.
There is therefore a genuine need for a device which processes a signal carrying information intended to be transmitted using a transmission terminal of radiofrequency signals which can enable an architecture to be designed which is capable of covering a wide band of frequencies, of supporting multiple types of modulation, and of reacting instantly to a large number of possible user requests.
In terms of performance, such an architecture will also be judged by its ability to supply sufficient power at the lowest possible level of energy consumption, since it is generally intended to be installed in a mobile terminal, by its ability to allow transmissions of high-bandwidth data, and by a high signal-to-noise ratio.
For example, polar loop architecture is very efficient in terms of energy consumption, since it requires only a moderately non-linear amplification with satisfactory efficiency in terms of power gain.
But in the case of broadband transmissions, a linear amplification architecture is generally unavoidable. Indeed, to obtain greater spectral efficiency, in this type of transmission the signals are modulated in terms of phase and amplitude according to the following polar representation: s(t)=A(t)·exp(j·(ωt+θ(t))). According to this polar representation, the expression A(t) is the, always positive, amplitude of the signal, the expression θ(t) is the phase and ω is the carrier. And variable amplitude signals generally require that linear-response amplifiers are used to prevent distortions. Since each amplifier is limited in terms of linearity by its AM/AM and AM/PM characteristics, this implies that amplifiers are used which operate in linear fashion in a zone sufficiently far removed from their saturation gain, and this makes the transmission system inefficient since these amplifiers are then less efficient in terms of energy consumption.
The architecture known by the name LINC (“Linear amplification using Nonlinear Components”) provides a solution to allow linear amplification of variable amplitude signals using amplifiers operating with gain saturation, i.e. with maximum efficiency for high power levels. This architecture also enables control loops, such as polar loops, to be avoided.
This architecture uses a device which processes in baseband a signal s(t)=A(t)·exp(j·θ(t)) carrying information intended to be transmitted using a radiofrequency signal, for the transformation of this signal into two signals of identical and constant amplitude AC, which are phase-shifted respectively relative to the information-carrying signal according to two variable and opposing phase-shifts:s1(t)=AC·exp(jφ(t))·exp(j·θ(t)), ands2(t)=AC·exp(−jφ(t))·exp(j·θ(t)).
Thus, as a vector representation, the information-carrying baseband signal s(t) is seen as the sum of these two constant amplitude AC signals, the opposing respective phase-shifts of which φ(t) and −φ(t) are a function of the variable amplitude of the information-carrying signal.
More specifically:
                    A        C            =                        max          ⁡                      [                          A              ⁡                              (                t                )                                      ]                          2              ,    and              φ      ⁡              (        t        )              =                            cos                      -            1                          ⁡                  [                                    A              ⁡                              (                t                )                                                    2              ⁢                                                          ⁢                              A                C                                              ]                    .      
Both these constant-amplitude signals can then be modulated (factor exp(j·ωt)) and amplified according to two independent modulation and amplification channels, before being recombined before transmission. There is no requirement that the amplification in each of the two channels should be linear, since neither of these signals carries any amplitude information. The amplifiers subjected to this technique are therefore advantageously used in their saturation zone in order to improve the overall efficiency of the radiofrequency transmission system.
Indeed, let G=GSAT·exp(j0) be the common transfer function of the amplifiers of the two modulation and amplification channels, at saturation.
At the output of the first modulation and amplification channel the first of the two constant-amplitude signals takes the following form:sO,1(t)=GSAT·s1(t)=GSAT·AC·exp(j·φ(t)+j·ω·t+j·θ(t)).
At the output of the second modulation and amplification channel, the second of the two constant-amplitude signals takes the following form:sO,2(t)=GSAT·s2(t)=GSAT·AC·exp(−j·φ(t)+j·ω·t+j·θ(t)).
By recombination of these two independently modulated and amplified signals, one obtains:sO(t)=sO,1(t)+sO,2(t), such thatsO(t)=GSAT·AC·exp(j·φ(t)+j·ω·t+j·θ(t))+GSAT·AC·exp(−j·φ(t)+j·ω·t+j·θ(t)),sO(t)=[GSAT·A(t)]·exp(j·(ωt+θ(t))), hencesO(t)=GSAT·s(t).
Amplification of the signal s(t) is therefore effectively linear, although this signal is variable in amplitude, and although the amplifiers are used with gain saturation.
In terms of power supplied compared to power consumed, if it is supposed that in all hypotheses a post-amplification filtering at −3 dB of losses is necessary and achievable, to supply for example a signal at +30 dBm to the transmission antenna, a single-amplifier architecture must supply a signal at +33 dBm at the output of the single amplifier, and at +27 dBm at the output of the single amplifier to supply, for example, a signal at +24 dBm. In order for the amplifier to operate in its zone of linearity, generally with a margin of 4 dB compared to its saturation gain, it must therefore be designed to support +37 dBm at saturation, if it is desired to transmit a signal at +30 dBm. Assuming 50% efficiency, this generally gives a consumption level of between 5 and 10 W, depending on the class of the amplifier. Such an architecture with this type of performance is, for example, described in the article by P. Wurm and A. Shirakawa, entitled “Radio transmitter architecture with all-digital modulator for opportunistic radio and modern wireless terminals”, CogART 2008, Proceedings on 2008 First International Workshop on Cognitive Radio and Advanced Spectrum Management, 14 Feb. 2008.
As a comparison, an LINC-type architecture with two amplification channels must provide a signal at +27 dBm (for φ=0) at the output of each amplifier in order to provide a signal at +30 dBm to the transmission antenna: indeed, in this architecture, the post-amplification filtering at −3 dB of losses can also perform the function of a module which recombines both the constant-amplitude signals. For φ=60° and for a signal at +27 dBm at the output of each amplifier, the signal supplied to the transmission antenna is at +24 dBm. Assuming an efficiency of 50%, this gives a consumption level of 2 W since the LINC architecture is still operating at saturation.
LINC architecture is therefore clearly advantageous since it provides a way of avoiding the traditional conflict between linearity and energy consumption which generally requires that compromises are made in the choice and design of the amplifiers.
Of course, this architecture theoretically requires that both the modulation and amplification channels are operating identically in order that the recombination of both constant amplitude signals allows, at the output of amplification, that the modulated information-carrying signal, which is amplified in the antenna of the radiofrequency transmission terminal, is regained.
In practice, LINC architecture poses a first problem since the transformation of the variable-amplitude information-carrying signal into two constant amplitude signals is not by its nature linear. This transformation broadens the spectrum of the transmitted signal, which pushes to their limits the capacities of the radiofrequency transmission system in terms of bandwidth, notably, for example, when considering broadband applications such as WCDMA applications. As a consequence, in the case of multi-standard applications, traditional LINC architecture rapidly reaches its limits.
Also in practice, LINC architecture is particularly sensitive to any gain or phase shift between the two modulation and amplification channels. And such shifts are inevitable, notably in the amplification part of these channels. It has been shown, for example, that a modulated OFDM signal, recombined after processing over two modulation and amplification channels, having a 5% gain shift (i.e. 0.42 dB) and a 3° phase shift, can have a spectrum which does not check the constraints of the OFDM spectral mask. As a consequence, LINC architecture requires very precise calibration of the two modulation and application channels.
Notably, in a range of possible values, the lower the amplitude of the information-carrying signal the more the calibration inaccuracies have consequences for its recombination.
For example, if the amplitude shift is zero between the two channels, i.e. if |sO,1(t)|=|sO,2(t)|=GSAT·AC (omitting the term exp(j·(ωt+θ(t))) in the equations, which does not impair the validity of the calculations, which relate to signal amplitudes), the amplitude of signal sO(t) is:−|sO(t)|=2·GSAT·AC for φ=0,−|sO(t)|=GSAT·AC for φ=60°, and−|sO(t)|=0.2·GSAT·AC for φ=84.26°,i.e. a dynamic of 20 dB between 0 and 84.26°.
If the shift is ±5% in terms of amplitude between the two channels, i.e. if |sO,1(t)|=1.05·GSAT·AC and |sO,1(t)|=0.95·GSAT·AC for example, the amplitude of signal sO(t) is:−|sO(t)|=2·GSAT·AC for φ=0,−|sO(t)|=1.0037·GSAT·AC for φ=60°, with a phase equal to arg [sO(t)]=4.94°, and−|sO(t)|=0.223·GSAT·AC for φ=84.26°, with a phase equal to arg [sO(t)]=26.4°, andi.e. a dynamic of 19 dB between 0 and 84.26°.
This example shows, firstly, that the dynamic is reduced by 1 dB due to the ±5% amplitude shift between the two channels and, secondly, that the consequences for the recombined signal are greater at low amplitude. Thus, at maximum amplitude (φ=0) the impact is zero, at half the maximum amplitude (φ=60°) the impact is limited, whereas at low amplitude, for example φ=84.26°, the impact becomes truly appreciable.
More generally, signals with a large dynamic will be affected by a change to a LINC architecture with imperfect calibration.